Darlington transistor circuit

ABSTRACT

A Darlington transistor circuit having a power transistor and a driver transistor is proposed. The two transistors are monolithically integrated in a common substrate (10) by a planar process, the substrate forming the collector zones of the two transistors. On the main surface of the substrate (10) there is a passivation layer (13) covering this main surface with the exception of contact windows. The base-collector junctions of the two transistors are protected by a metal electrode (15), which is located above the passivation layer (13) and extends up to a stop ring (14), which is disposed beneath the passivation layer (13) in the substrate (10). The potential at the cover electrode (15) is adjustable with the aid of a voltage divider (16). (FIG. 3).

BACKGROUND

The invention relates generally to a Darlington transistor circuit, andmore particularly to a monolithic integrated version thereof.

Circuits of this kind are already known. However, they have thedisadvantage that electrical fields acting upon them from outside, suchas are produced by the polarization of photoresist coatings duringoperation at high voltage and temperature, can cause the degradation ofblocking-state characteristic curves, or else that the breakdown voltagecan be influenced within certain limits only by varying the thickness ofthe passivation layer but is not variable after the metallizing coatinghas been applied.

THE INVENTION

Briefly, the Darlington transistor circuit according to the inventionfeatures a cover electrode over the doped semiconductor material and apassivation layer insulating the cover electrode from the semiconductorjunctions and has the advantage over the prior art that the space chargezone forming around the first and second zones during operation islimited to inside the annular zone serving as a stop ring and underneaththe cover electrode is shielded from external electrical fields. Afurther advantage is that the breakdown voltage at the first and secondp-n junctions is variable within wide limits. Providing indirectconnections between the cover electrode, a common collector portion ofthe semiconductor material, and one of the base zones in thesemiconductor material affords a particularly simple opportunity forrealizing the setting of this breakdown voltage with the aid of avoltage divider. Particularly advantageous further developments of thesubject of claims 1 and 2 are disclosed in the further dependent claims3-12.

DRAWING

Exemplary embodiments of the Darlington transistor circuit according tothe invention are shown in the drawing and explained in greater detailin the ensuing description. Shown are:

FIG. 1, the electrical circuit diagram of a known Darlington transistorcircuit comprising two n-p-n transistors;

FIG. 2, a section taken through the layout of a Darlington transistorcircuit according to FIG. 1;

FIG. 3, a partial section through a Darlington transistor circuitaccording to the invention, having an external ohmic voltage divider foradjusting the breakdown voltage at the first and second p-n junctions;

FIG. 4, the breakdown voltage U_(Br) at the first p-n junction inaccordance with the divider ratio of this external voltage divider;

FIG. 5, a plan view on a Darlington transistor circuit according to theinvention, in which the ohmic voltage divider is monolithicallyintegrated, having closed contact windows;

FIG. 6, a plan view on the disposition of FIG. 5, but with openedcontact windows;

FIG. 7, a section taken along the line A-A' of FIG. 6;

FIG. 8, a plan view on a portion of a Darlington transistor circuitaccording to the invention, in which the ohmic voltage divider ispartially replaced by a chain of Zener diodes; and

FIG. 9, a section taken along the line B-B' of FIG. 8.

DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

FIG. 1 shows the electrical circuit diagram of a Darlington transistorcircuit having an n-p-n power transistor T₁ and an n-p-n drivertransistor T₂ and with respective resistors R₁ and R₂ parallel to theemitter-base paths of these two transistors.

In FIG. 2, a section through the layout of a Darlington transistorcircuit according to FIG. 1 is shown. In the common n-conductivesemiconductor substrate 10, the two p-conductive base zones 11a and 11bof the two transistors T₁, T₂ are diffused in from a main surface. Thesezones, together with the semiconductor substrate 10, form p-n junctions12a and 12b. From the same main surface, the corresponding n-conductiveemitter zones 9a and 9b of the two transistors T₁, T₂ are diffused intothe zones 11a and 11b. A passivation layer 13 covers those parts of thismain surface at which p-n junctions meet the surface. The externalconnections of the Darlington transistor circuit are marked E, B and C,as in FIG. 1, with E being the emitter connection, B the base connectionand C the collector connection. Between the metallizing coating for theemitter connection E and the metallizing coating for the base connectionB, on this same main surface, there is a connecting metallizing coating8 for connecting the emitter zone 9b of the driver transistor T₂ withthe base zone 11a of the power transistor T₁.

The planar p-n junctions 12a and 12b are now supposed to be protected bya metal electrode 15 via the insulator 13 such that the blockingbehavior cannot be affected undesirably by external influences (such assubstances having polar groups, alkali ions, metal flakes and so forth).

FIG. 3 shows an exemplary embodiment in a schematic illustration. Thesubstrate 10 is of n⁻ -conductive silicon, and the insulator 13 is ofthermally grown silicon dioxide. The metal electrode 15 overlaps the pzone 11a and the p zone 11b (not shown) as well as an n⁺ region 14,which is diffused in simultaneously with the emitters of the twotransistors T₁ and T₂. The n⁺ -conductive emitter zone 9a of the n-p-ntransistor T₁ is not shown in FIG. 3, however. The attainable breakdownvoltage at the p-n junctions 12a and 12b depends not only on the basicdoping of the silicon but also, substantially, on the thickness of theoxide layer 13 and on the potential of the electrode 15.

FIG. 4 shows the dependency of the breakdown voltage as a function ofthe divider ratio R₁ :R of an external voltage divider 16. U₁ is thebreakdown voltage at the p-n junction 12a which is obtained if the coverelectrode 15 is connected directly to the p zone 11a, without theinterposition of the resistor R₁. In the present example, U₁ is markedlylower than the breakdown voltage attainable without a cover electrode15. It is equal to the voltage at the depletion breakdown of acorresponding MOS structure. U₂ is the breakdown voltage obtained if thecover electrode 15, dispensing with the voltage divider 16, is connectedto the n⁺ region 14 or to the common collector zone of the two n-p-ntransistors T₁ and T₂. Because of the increase in field intensitydictated by the charge carrier enrichment, U₂ is likewise lower than thebreakdown voltage without a cover electrode 15. The highest attainablebreakdown voltage amounts to U₁ +U₂, at a divider ratio of R₁ :R=U₂ :(U₁+U₂). The temperature range of the breakdown voltage is somewhat smallerin a transistor than in a Zener diode with the same blocking voltage,especially if the voltage divider 16 is adjusted to the right flank ofthe voltage curve in FIG. 4.

FIGS. 5 and 7 show one example of how the voltage divider 16 can bemonolithically integrated. As shown by the plan view in FIG. 5, thevoltage divider resistor 16a forms an elongated p-conductive zone 116adiffused in from the main surface of the substrate 10. This zone 116acomprises an extension of the second zone 11b and is covered at leastpartially by the passivation layer 13 and by the metallizing coatingextending beyond it and serving as the cover electrode 15. The edge ofall the metallizing coatings is indicated in FIGS. 5 and 6 by dashedlines. At a specific location over the zone 116a, the passivation layer13 under the cover electrode 15 is removed. The portion of the coverelectrode 15 coming to rest inside the thus-formed contact window 122athereby forms the tap 16b of the voltage divider 16. The first end 19 ofthe voltage divider resistor 16a is located at the lower right corner ofthe second zone 11b in FIGS. 5 and 6 and establishes an uninterruptedtransition from the second zone 11b to the elongated zone 116a. It wouldalso be conceivable, and in certain applications advantageous as well,to provide a direct connection of the zone 116a with the zone 11a. Thesecond end 20 of the voltage divider resistor 16a is formed by the otherend of the zone 116a, which in FIGS. 5 and 6 is located somewhat to theright of the lower left corner of the substrate 10. This second end 20of the voltage divider resistor 16a formed by the zone 116a is connectedin such a manner with the stop ring 14, by means of a metallizingcoating 22 extended beyond the passivation layer 13, that thismetallizing coating 22 is bonded, via contact windows 22a disposed inthe passivation layer 13, to the regions located beneath them which areto be connected with one another.

Above the upper left portion of the zone 116a, a recess which exposesthe passivation layer 13 is disposed in the cover electrode 15. Contactwindows 301, 302, 303 are introduced into the portion of the passivationlayer 13 exposed by the recess 30 and extend in a row in thelongitudinal direction of the elongated zone 116a. These contact windows301, 302, 303 are bridged over by the short-circuit metallizing coatingM, which short-circuits the portions of the third zone 116a locatedbetween these contact windows 301, 302, 303. The voltage divider 16 canbe calibrated by splitting off individual parts M₁, M₂ of thisshort-circuit metallizing coating M, in order to be able to set thebreakdown voltages at the p-n junctions 12a and 12b to a desired value.A suitable method for splitting off metallizing bridges of this kind isdescribed in German Examined Application DE-AS No. 22 56 688.

With the disposition shown in FIGS. 5-7, the overall result attained isthat all the regions bordering on the main surface discussed above, inwhich space charging takes place when a blocking voltage is applied tothe p-n junctions 12a, 12b--that is, essentially regions of zone 10 andan adjacent strip of zones 11a, 11b--are protected by means of ametallic cover electrode 15 located over the silicon dioxide layer 13.

A C-shaped part 10a of zone 10, which partially separates the base zones11a, 11b of the power and driver transistors from one another can becovered, without any disadvantageous effect on blocking ability, by themetallizing coating 8, which is connected to the base zone of the powertransistor T₁ and to the emitter zone of the driver transistor T₂.

Two webs 11c and 11d which are of base material and connect the zones11a and 11b with one another act as a resistor between the base zones11a and 11b of the two transistors T₁ and T₂ which is labelled R₂ inFIG. 1. Should this resistor not be desired, then the C-shaped zone 10acan be extended to form a ring, which is likwise capable of beingprotected completely by a covering metallizing coating.

For the sake of simplicity, details which have nothing to do with theconcept of the invention have not been shown. Such details are, forinstance, the serrations of emitter zones, a resistor R₁ between theemitter and the base of the power transistor T₁, a diode parallel to theemitter-collector path of the power transistor T₁ and an overlapping ofthe emitter-base junctions on the part of the metallizing coating.

In FIGS. 5 and 6, in the area around the metal strip 22, the boundary ofthe n⁺ zone is selected such that the n⁻ zones (zone 10) are coveredcompletely everywhere, even at the less-critical points, by the metalelectrode 15.

An n⁺ strip 114, which communicates with zone 14, is located between thep zone 11a or 11b, respectively (the base zone) and the p zone 116a(voltage divider resistor). This strip 114 prevents the establishment ofa connection between the zones 11a, 11b and the zone 116a via the spacecharge if a blocking voltage is applied to the p-n junctions 12a and12b.

In order to attain high voltage divider resistances while requiringlittle surface area, it is recommended that an additional photo processbe provided for the zone 116a of the voltage divider, in order toundertake a lesser doping of impurities independently of the base zones11a and 11b.

In the exemplary embodiment of FIGS. 8 and 9, the portion located in theupper left corner of FIGS. 5 and 6 of the ohmic voltage resistor 16aformed by the zone 116a is replaced by a chain of Zener diodes Z₁, Z₂,Z₃. These Zener diodes each comprise a zone 40 of p-type conductivitydiffused into the main surface of the substrate 10 and a zone 41 of n⁺-type conductivity diffused into this zone 40. In the vicinity of theZener diodes Z₁, Z₂, Z₃ a tongue-like recess 31 is provided in themetallizing coating 15, and the Zener diodes Z₁, Z₂, Z₃ are connectedwith one another inside this recess 31 by means of metallizing bridgesB₁, B₂. Inside the recess 31, the last Zener diode Z₃ of the chain Z₁,Z₂, Z₃ is furthermore connected, by means of a further metallizingbridge B₃, with a portion of the stop ring 14. The connections are eachrealized such that the metallizing bridges B₁, B₂, B₃ are bonded viacontact windows disposed in the passivation layer 13 to thecorresponding areas located beneath them.

In the exemplary embodiment shown in FIGS. 8 and 9, in which a portionof the ohmic voltage divider resistor 16a has been replaced by a chainof Zener diodes Z₁, Z₂, Z₃, the p-conductive zones 40 of these Zenerdiodes are diffused in at the same time as the p zones 11a and 11b andthe p zone 116a; the n⁺ zones 41 are diffused in simultaneously with then⁺ zone 14 and the n⁺ -conductive emitter zones 9a, 9b of the twotransistors T₁ and T₂. The breakdown voltage of the Zener diodes Z₁, Z₂,Z₃ is on the order of magnitude of 10 volts.

In order to adjust the blocking voltage of the main junctions 12a and12b, the Zener diodes Z₁, Z₂, Z₃ can be short-circuited individually,perhaps by the brief application of a high blocking voltage, combinedwith a high blocking current.

Naturally the number of calibration paths is not limited to that of theexemplary embodiment shown. The calibration processes described can alsobe combined with one another as needed.

I claim:
 1. An external-field-insensitive Darlington transistor circuit,with adjustable junction breakdown voltage, having a power transistor(T1) and having a driver transistor (T2), the collector of which drivertransistor is connected to the collector of the power transistor (T1)and the emitter of which driver transistor is connected to the base ofthe power transistor (T1), and in which circuit the power transistor(T1) and the driver transistor (T2) are monolithically integrated by aplanar process in a common substrate (10) forming the collector zones ofthe two transistors (T1, T2),wherein a first (11a) and a second zone(11b) are diffused into the substrate (10) from a main surface, thesubstrate (10) having a specified conductivity type and the first (11a)and second (11b) zones having the opposite conductivity type, said firstand second zones (11a, 11b) respectively forming with the material ofthe substrate (10) a first (12a) and a second (12b) p-n junction and thebase zones of the two transistors (T1, T2), wherein, from the same mainsurface of the substrate (10), an emitter zone, having the sameconductivity type as the basic material of the substrate (10) but ahigher concentration of impurities, is diffused into each of said firstand second base zones (11a, 11b), said emitter zones serving as theemitter zones (9a, 9b) of the respective transistors, and wherein asubstantially non-conductive passivation layer (13) covering this samemain surface of the substrate (10), except for contact windows, isprovided, comprising a further annular zone (14), serving as a stopring, diffused into the substrate (10) from the same main surface aroundthe first (11a) and second (11b) base zones, said annular stop ring zone(14) having the same conductivity type as the basic material of thesubstrate (10) but a higher concentration of impurities; a metalcoating, serving as a cover electrode (15) and as a shield againstexternal electric fields, applied atop the passivation layer (13), saidmetal coating (15) extending over a region between the base zones (11a,11b) and the annular stop ring zone (14), and overlapping not only theannular stop ring zone (14), but also portions of the base zones (11a,11b) adjacent to the annular stop ring zone (14), and a voltage divider(16) monolithically integrated with said power (T1) and driver (T2)trransistors in said circuit, for adjusting the potential of the coverelectrode (15) to a value dependent upon the desired breakdown voltageof the first and second p-n junction (12a, 12b), which value falls in arange defined by the potential of the first base zone (11a) and thepotential of a portion of the substrate (10) located outside the first(11a) and second (11b) zones and having a conductivity type oppositefrom that of the first (11a) and second (11b) base zones, said voltagedivider (16) including a voltage divider resistor (16a) and a tap (16b),and PG,14 wherein the tap (16b) is connected to the cover electrode(15), the first end (19) of the voltage divider resistor (16a) isconnected with one of the first zone (11a) and the second zone (11b),and the second end (20) of the voltage divider resistor (16a) isconnected on a surface of said integrated circuit with a portion of thesubstrate (10) located outside the first (11a) and second (11b) zones,which portion has a conductivity type opposite from that of the first(11a) and second (11b) zones, whereby said breakdown voltage can beadjusted, after fabrication of said integrated circuit, by varyingwhere, on the surface thereof, one of said ends of said voltage dividerresistor (16a) is connected.
 2. A Darlington transistor circuit asdefined by claim 1, characterized in that the voltage divider resistor(16a) includes at least one ohmic resistor.
 3. A Darlington transistorcircuit as defined by claim 2, characterized in that the voltage dividerresistor (16a) includes an elongated zone (116a) diffused into thesubstrate (10) from the same main surface and having the sameconductivity type as the first (11a) and the second (11b) zone, whereinthe elongated zone (116a) forms an extension of the first (11a) or thesecond (11b) zone and is covered at least partially by the passivationlayer (13) and by the metallizing coating extending over and beyond itand serving as the cover electrode (15), and that at a specific locationabove the elongated zone (116a) the passivation layer (13) is removedunderneath the metallizing coating serving as the cover electrode (15),whereupon the portion of the cover electrode (15) coming to be locatedinside the thus-formed contact window (122a) forms the tap (16b) of thevoltage divider (16).
 4. A Darlington transistor circuit as defined byclaim 3, characterized in that the voltage divider (16) is a purelyohmic voltage divider and that the elongated zone (116a) forms theentire voltage divider resistor (16a).
 5. A Darlington transistorcircuit as defined by claim 4, characterized in that the second end (20)of the voltage divider resistor (16a) formed by the elongated zone(116a) is connected in such a manner, by means of a metallizing coating(22) extended over and beyond the passivation layer (13), with a portionof the substrate (10) which has a conductivity type opposite from thatof the first (11a) and the second (11b) zones, preferably with a portionof the stop ring (14), that this metallizing coating (22) is bonded viacontact windows (22a) disposed in the passivation layer (13) to theregions (20, 14) located therebeneath and which are to be connected withone another.
 6. A Darlington transistor circuit as defined claim 3,characterized in that over a portion of the elongated zone (116a) in themetallizing coating serving as the cover electrode (15), a recess (30)is disposed which exposes the passivation layer (13), that contactwindows (301, 302, 303) are introduced into the portion of thepassivation layer (13) exposed by the recess (30), which contact windowsextend in a row in the longitudinal direction of the elongated zone(116a), that these contact windows (301, 302, 303) are bridged over by ashort-circuit metallizing coating (M), which short-circuits the portionsof the elongated zone (116a) located between the contact windows (301,302, 303), and that the voltage divider (16) is calibratable by thesplitting off of individual portions (M₁, M₂) of this short-circuitmetallizing coating (M), in order to adjust the breakdown voltage at thefirst p-n junction (12a) to a desired value.
 7. A Darlington transistorcircuit as defined by claim 2, characterized in that the voltage dividerresistor (16a) comprises an ohmic resistor and a chain of Zener diodes(Z₁, Z₂, Z₃) connected in series with this resistor and with oneanother.
 8. A Darlington transistor circuit as defined by claim 3,characterized in that the Zener diodes (Z₁, Z₂, Z₃) each comprise a zone(40) diffused into the same main surface of the substrate (10) andhaving the same conductivity type as the first (11a) and the second(11b) zones and a zone (41) diffused into this zone (40) and having thesame conductivity type as the basic material of the substrate (10) but ahigher concentration of impurities, that in the vicinity of the Zenerdiodes (Z₁, Z₂, Z₃) a recess (31) is provided in the metallizing coating(15) serving as the cover electrode, inside which recess (31) the Zenerdiodes (Z₁, Z₂, Z₃) are connected with one another by means ofmetallizing bridges (B₁, B₂) and inside which recess (31) the last Zenerdiode (Z₃) of the chain (Z₁, Z₂, Z₃) is connected by means of a furthermetallizing bridge (B₃) with a portion of the substrate (10) which has aconductivity type opposite that of the first (11a) and second (11b)zones, preferably with a portion of the stop ring (14), whereupon thesemetallizing bridges (B₁, B₂, B₃) are bonded via contact windows disposedin the passivation layer (13) to the corresponding regions locatedtherebeneath;wherein the voltage divider resistor (16a) comprises anohmic resistor and a chain of Zener diodes (Z₁, Z₂, Z₃) connected inseries with the ohmic resistor and with one another.
 9. A Darlingtontransistor circuit as defined by claim 8, characterized in that thevoltage divider (16) is calibratable by means of the short-circuiting ofindividual Zener diodes (Z₁, Z₂, Z₃) in order to adjust the breakdownvoltage at the first p-n junction (12a) to a desired value.
 10. ADarlington transistor circuit as defined by claim 9, characterized inthat the Zeher diodes (Z₁, Z₂, Z₃) are short-circuitable by means of thebrief application of a high blocking voltage, combined with a highblocking current.
 11. A Darlington transistor circuit as defined byclaim 4,characterized in that over a portion of the elongated zone(116a) in the metallizing coating serving as the cover electrode (15), arecess (30) is disposed which exposes the passivation layer (13), thatcontact windows (301, 302, 303) are introduced into the portion of thepassivation layer (13) exposed by the recess (30), which contact windowsextend in a row in the longitudinal direction of the elongated zone(116a), that these contact windows (301, 302, 303) are bridged over by ashort-circuit metallizing coating (M), which short-circuits the portionsof the elongated zone (116a) located between the contact windows (301,302, 303), and that the voltage divider (16) is calibratable by thesplitting off of individual portions (M₁, M₂) of this short-circuitmetallizing coating (M), in order to adjust the breakdown voltage at thefirst p-n junction (12a) to a desired value.
 12. A Darlington transistorcircuit as defined by claim 5,characterized in that over a portion ofthe elongated zone (116a) in the metallizing coating serving as thecover electrode (15), a recess (30) is disposed which exposes thepassivation layer (13), that contact windows (301, 302, 303) areintroduced into the portion of the passivation layer (13) exposed by therecess (30), which contact windows extend in a row in the longitudinaldirection of the elongated zone (116a), that these contact windows (301,302, 303) are bridged over by a short-circuit metallizing coating (M),which short-circuits the portions of the elongated zone (116a) locatedbetween the contact windows (301, 302, 303), and that the voltagedivider (16) is calibratable by the splitting off of individual portions(M₁, M₂) of this short-circuit metallizing coating (M), in order toadjust the breakdown voltage at the first p-n junction (12a) to adesired value.